The genome is a highly sensitive organ of the cell that monitors its activities and corrects common errors, senses unusual and unexpected events, and responds to them, often by restructuring. ~ American cytogeneticist Barbara McClintock
A genome is (the idea of) the total complement of genes in a cell or organism. If a gene is a recipe, a genome is a recipe book.
Different cell types express different portions of their genome. ~ Gordon Tomkins et al in 1969
It was long supposed that all cells in an organism had the same genome, as the above quote suggests. But that is not so. Multicellular organisms comprise a population of cells, each with its own personal genome (pergenome). Even cells of the same type have their own pergenome.
Prokaryotes have a flexible genome that can change during a single life cycle. This can happen because prokaryotes can readily pick up new genetic material.
Chromosomal mosaicism – genetic variation among cells – can occur by a variety of means, including errors during chromosome segregation or DNA replication, copying variations, gene rearrangement, single-nucleotide variation, or other instabilities.
Such mutations can occur at any stage of development: in stem cells, differentiating cells, and in somatic cells (which are nominally terminally differentiated). The genetic makeup of a multicellular organism is multifarious.
Over evolutionary time, all organisms selectively incorporate alien genetic material. Human DNA includes gene packages from at least 8 retroviruses. Some of these viral genes are essential to human reproduction.
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A genome comes in no particular order. While genome structure is surmised as significant, it is more likely to have been preserved simply by inertia.
Intuitively, you wouldn’t believe that just by chance things would be conserved for 500 million years. ~ French molecular biologist Daniel Chourrout
The number of genes in an organism is a meaningless statistic, especially in comparing organisms in the same kingdom. Some prokaryotes have thousands of genome copies (polyploidy) .
For multicellular eukaryotes, only a fraction of a genome is actively employed. Most of a genome is kept as a historical reference: a database of possibilities for the future from the experiences of the past. This legacy information is accessed as needed.
Plants commonly experiment genetically. They may duplicate their genome, with the original serving as a reference, and the copy as a testbed. For instance, 70 million years ago, the tomato triplicated its genome: keeping a preserved master copy and generating 2 spare copies to adaptively mutate. One result was the birth of the potato, a tuber-producing evolutionary offspring.
Replication is like a mirror that reflects the evolutionary history of living beings: the first genes to be replicated are the oldest, while those that replicate later on are the youngest. ~ Spanish biologist Alfonso Valencia
In replicating a genome, the most valued, conserved genes are copied first. Newer genes, in evolutionarily active regions, are copied afterwards.
The regions that replicate late also have a compact and inaccessible structure; they are hidden zones in the genome that act as evolutionary laboratories, where these genes can acquire new functions without affecting essential processes in the organism. ~ Spanish biologist David de Juan